1. Field of the Invention
The present invention relates to a method for evaluating the reliability of electrical power measuring devices.
2. Description of the Related Art
In the past, plasma processing systems have been developed that process articles such as semiconductor wafers or liquid crystal substrates using a method such as etching by supplying high-frequency electrical power output from a high-frequency power supply device to a plasma processing device. In these plasma processing systems, since there is the risk of fluctuations in an impedance of the plasma processing device causing a reflected power reflected at the input end of the plasma processing device to damage the high-frequency power supply device, an impedance matching device is typically provided between the high-frequency power supply device and the plasma processing device, and the matching operation of the impedance matching device is controlled corresponding to fluctuations in the impedance of the plasma processing device, or the impedance of the plasma processing device or high-frequency voltage and high-frequency current and the like at the input end of the plasma processing device is monitored (see Japanese Patent Application Laid-open No. 2007-163308).
Monitoring the matching operation of the impedance matching device or the plasma processing device is carried out by providing a high-frequency measuring device on the output end of the impedance matching device and the input end of the plasma processing device, detecting high-frequency voltage (to be simply referred to as “voltage”) and high-frequency current (to be simply referred to as “current”) with the high-frequency measuring device, and in addition to determining a phase difference between the voltage and current (to be simply referred to as “phase difference”) θ from the detected values, calculating high-frequency parameters such as an effective voltage value V, an effective current value I, an impedance Z=R+jX of the plasma processing device, a reflection coefficient Γ, a forward power Pf input to the plasma processing device, and a reflected power Pr reflected at the input end of the plasma processing device due to impedance mismatch, and then using those high-frequency parameters.
The high-frequency measuring device is provided with a capacitor capacitatively coupled to a rod-shaped semiconductor for transmitting electrical power to the plasma processing device and a coil magnetically coupled to the body portion thereof, and together with detecting a voltage v=√{square root over (2)}·V·sin(ωt) with the capacitor or a current i=√{square root over (2)}·I·sin(ωt+θ) with the coil, a phase difference θ is determined from these detected values, and the high-frequency parameters are calculated according to equations (1) to (5) below using the voltage v, the current i and the phase difference θ. Namely, the high-frequency measuring device is referred to as a so-called RF sensor provided with sensors for detecting voltage v and current i, and an arithmetic processing circuit for calculating the high-frequency parameters from the detected values of those sensors.
                    R        =                              V            I                    ⁢          cos          ⁢                                          ⁢          θ                                    (        1        )                                X        =                              V            I                    ⁢          sin          ⁢                                          ⁢          θ                                    (        2        )                                Z        =                  R          +          jX                                                                    Γ        =                                                            (                                                                            R                      2                                        +                                          X                      2                                        -                    1                                                                                                      (                                                  R                          +                          1                                                )                                            2                                        +                                          X                      2                                                                      )                            2                        +                                          (                                                      2                    ⁢                                                                                  ⁢                    X                                                                                                      (                                                  R                          +                          1                                                )                                            2                                        +                                          X                      2                                                                      )                            2                                                          (        3        )                                Pf        =                              VI            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                                1            -                          Γ              2                                                          (        4        )                                Pr        =                  Pf          ⁢                                          ⁢                      Γ            2                                              (        5        )            
Since values detected with sensors differ from the correct values due to variations in sensor sensitivity, monitoring devices and measuring devices are typically composed to acquire calibration data that converts detected values to correct values by preliminarily measuring a measured object serving as a reference, and then correcting detected values to correct detection values with the calibration data during actual measurement.
In the case of calibrating high-frequency measuring devices, for example, a high-frequency measuring device is arranged between a dummy load serving as a reference measured object having a characteristic impedance of the measuring system (a characteristic impedance of the transmission line over which high-frequency waves are transmitted for measurement; normally 50Ω or 75Ω) and a high-frequency power supply device, and calibration data is acquired for detected voltage values and detected current values of the high-frequency measuring device when a prescribed high-frequency electrical power is supplied from the high-frequency power supply device to the dummy load.
However, in a plasma processing system, since the load to which high-frequency electrical power is supplied from the high-frequency power supply device is plasma generated within a plasma processing device, that impedance is frequently a complex impedance having strong reactance. Although high-frequency electrical power PL actually input to the plasma processing device (to be referred to as “effective power PL”) is represented as PL=Pf−Pr, as is clear from the above-mentioned equations (4) and (5), the effective power PL is calculated by PL=V·I·cos θ. According to this equation, it is difficult to correctly calculate the effective power PL supplied to a load having a complex impedance unless the effective voltage value V, the effective current value I and the phase difference θ are each calculated correctly. In particular, in a plasma processing system having for the load a plasma processing device in which the load impedance has a phase difference θ close to 90° resulting in a complex impedance having large reflection, since the error in the effective power PL becomes extremely large even if there is only a slight error in the phase difference θ, it is difficult to measure effective power PL with high reliability in high-frequency measuring devices.
In a system in which high-frequency electrical power is supplied from a high-frequency power supply device to a load having a complex impedance with extremely large reflection in the manner of a plasma processing system, when the reliability of a measured value of effective power PL supplied to the load by a high-frequency measuring device is attempted to be evaluated, although it is necessary to employ a method in which, for example, a measured value is set for effective power PL that serves as a reference when a prescribed high-frequency electrical power is supplied to a load having a complex impedance, and a measured value of the effective power PL of the high-frequency measuring device is evaluated by comparing with the reference measured value, such a method for evaluating the reliability of an electrical power measured value of a high-frequency measuring device has yet to be proposed.
Consequently, there has previously been the problem of being unable to evaluate the reliability of high-frequency measuring devices for measuring the effective power input to a load having a complex impedance. In addition, since criteria for evaluating reliability during manufacturing of high-frequency measuring devices are not clearly defined, it was also difficult to inspect for defective products.